The need to reduce fossil fuel consumption and emissions in automobiles and other vehicles predominately powered by internal combustion engines (ICEs) is well known. Vehicles powered by electric motors attempt to address these needs. Another alternative solution is to combine a smaller ICE with electric motors into one vehicle. Such vehicles combine the advantages of an ICE vehicle and an electric vehicle and are typically called hybrid electric vehicles (HEVs).
The desirability of combining an ICE with electric motors is clear. There is great potential for reducing vehicle fuel consumption and emissions with no appreciable loss of vehicle performance or drive-ability. The HEV allows the use of smaller engines, regenerative braking, electric boost, and even operating the vehicle with the engine shutdown. Nevertheless, new ways must be developed to optimize the HEV's potential benefits.
An HEV can use batteries to store electrical energy for use by the vehicle's electric motor. Active control of the HEV battery becomes a critical vehicle function to achieve the HEV goals of reduced emissions and fuel economy. Such active battery control cannot only increase overall vehicle performance and fuel economy but also increase battery life.
Typical batteries installed in the hybrid vehicles are discharged during a high-load operation such as acceleration and recharged during a low-load operation such as traveling at constant speed or deceleration. In order to carry out such discharging and recharging effectively, it is important to keep SOC (State Of Charge which is also referred to as an available reserve capacity or residual electric energy) at a middle value (e.g., 50% to 70% of a fully charged energy of a battery). It is, thus, essential to monitor the SOC of the storage battery.
As a method of measuring the SOC, there is known a technique for integrating or totalizing the amount of current discharged from a storage battery. This technique, however, encounters a drawback in that errors in totalizing the discharged current are accumulated due to a variation in charging/discharging efficiency, thus making it difficult to measure the amount of reserve current in the battery accurately. In order to avoid this problem, Japanese Patent First Publication No. 2000-69606 proposes a correction system designed to correct the charging/discharging efficiency as a function of a difference between an actual state of charge and an estimated state of charge of a storage battery. The actual state of charge is determined by an upper or a lower limit of a voltage-to-current characteristic stored in a memory of the system when it is reached. The estimated state of charge is determined by a totalized amount of current discharged from the battery. The system, however, has a problem in that it is difficult to eliminate the totalizing error completely because of a change in charging/discharging efficiency arising from the history of use of the battery.
Battery availability and life in hybrid applications are highly dependent on proper management of the battery state of charge. Current methods using principally current integration, is not robust to sensor noncompliance, memory effect, battery age, or temperature. In addition, vehicle testing has demonstrated that lookup tables are not robust to any of hysteresis, memory effect, battery age, temperature, or sensor noncompliance. Improper state of charge management can lead to accelerated cell degradation, degraded vehicle performance requirements and inadequate over-charge or over-discharge protection.
Battery state of charge (SOC) controls for vehicles are known in the prior art using various conditions or criteria. U.S. Pat. No. 6,629,027 to Yamaguchi et. al. detects a state of charge from an integral of a voltage with respect to time and obtains an electric power output. Additionally, U.S. Pat. No. 6,091,228 to Chady et al. uses current integration and produces a signal source representing the desired auxiliary source current.
However each of the existing battery SOC controls do not calculate a state of charge based on a linear region of an average open-circuit voltage that operates to eliminate the effects of hysteresis and internal battery resistance.
Therefore it is desirable to provide a control system and method that corrects a current-based state of charge calculated by providing an offset correction value based on an average open circuit voltage that operates to eliminate SOC calculation errors caused by sensor drift, sensor noncompliance, hysteresis, and effects of battery age.